The present invention relates to methods and apparatus for patterning articles such as electronic, e.g. photovoltaic devices and methods and apparatus for their construction.
Electronic devices such as photovoltaic thin film modules consist of functional layers of a defined geometry that are positioned with respect to one another such that a photovoltaic module is formed. These functional layers are typically charge carrier extraction layers, photoactive layers for accepting electromagnetic radiation and outputting electrical energy and electrode layers.
A photovoltaic module consists of a number of individual photovoltaic cells connected together. These individual cells may be connected in series or in parallel to each other. Higher output voltages may typically be achieved by electrically connecting the cells in series.
Photovoltaic modules such as thin film photovoltaic modules are sometimes fabricated as a monolithic array of cells. Rather than wiring individual cells together (as is sometimes performed for, say, crystalline solar modules), the monolithic interconnection of thin film modules may be achieved either by producing a substrate substantially covered by the necessary functional layers and patterning those functional layers to produce a network of individual cells on the substrate or by directly printing the functional layers to the substrate in the desired pattern.
As is clear to the skilled person, the relative ease of manufacture of a monolithic module provides a cheaper alternative to electrically connecting an array of individual wafers. However, known methods for producing monolithic arrays have limitations which may have a detrimental effect on the quality of finished photovoltaic modules.
In order to maximise the power conversion efficiency of photovoltaic modules, it is necessary to ensure a high homogeneity of the film thickness, that thickness usually being in the order of ten to several hundred nanometers for the photoactive layer of organic photovoltaic cells and in the order of microns for inorganic thin film cells, and a high resolution edge definition.
Printing techniques such as gravure printing enable the deposition of a patterned thin film in a single process step. However, not all layers of a device can be deposited by printing techniques. For example, some vacuum deposited transparent electrodes (ITO) or highly reflective metal electrodes may not be deposited by printing techniques. The highest film quality with regard to a high homogeneity and a low defect density (pinholes) is likely to be achieved by coating techniques rather than by printing techniques like gravure printing. Due to the low solid content of the solutions caused by the limited solubility of some of the components, the cell volumes in the gravure structure of printing cylinders must be relatively large to produce functional layers of the required thickness. High cell volumes result in large cells in the order of several tens of micrometers, meaning that the edge resolution of gravure to non-gravure areas is limited. As a consequence the formation of straight uniform printing edges on the order of micrometers is unlikely. In addition, the thickness of a printed area will always vary starting from the edge towards the centre of the printed layer. The length of this variation depends on the rheological properties of the material, the drying conditions and the surface tension of the substrate. A deviation from the optimum film thickness will result in lower power conversion efficiencies and is therefore unfavourable. Accordingly, coating or otherwise providing large areas will lead to a greater degree of homogeneity in a finished article.
Coating techniques such as slot-die coating or blade coating typically offer highly uniform functional layers. However these layers are either not patterned or include patterning with coarse resolutions, e.g. requiring mm scales for stable gap sizes in the slot-die coating process. Thus post-processing is usually required to produce the required array of cells on the substrate.
Further, patterning steps include laser scribing which may be useful for patterning organic and inorganic layers in e.g. organic solar cell modules.
Laser scribing uses one or more lasers to ablate the functional layers of the substrate to create the desired pattern. However, typically the laser ablates all of the functional layers on the substrate (and might also damage or otherwise affect the substrate), where it may actually be desired to ablate only one or some of the layers.
While the selective patterning of only certain layers by laser scribing is possible, it relies, particularly when the layers are thin, upon the individual layers having sufficiently distinct absorption characteristics and the according use of lasers of different wavelengths to ablate each layer. Moreover, lasers are likely to damage the underlaying barrier layers (for example layers of SiN, Al2O3 or SiO2 between the substrate and electrode), and the equipment is relatively expensive, especially where a plurality of lasers (or indeed tunable lasers) are required.
Mechanical scribing, such as the use of hardened steel tips to remove layers from a thin film, is also used to pattern, e.g., inorganic thin film modules and organic photovoltaic modules. However significant disadvantages include the destruction of underlying surfaces, remaining traces of organic components (which may be critical in areas that will be utilized for electrical interconnects, as a subsequently coated electrode layer may suffer from a high electrical interface resistance caused by partially removed organic layers), and the production of particles that may cause damage in the subsequent deposition and encapsulation processes.